In typical flow cytometers, the determination procedure is a simple counting procedure where a part of a milk sample is mixed with dye and the number of cells therein is counted using e.g. a fluorescence measurement.
The uncertainty of a measurement can be expressed by the standard deviation (cf. D. A. Skoog `Principles of instrumental analysis`, Saunders College Publishing, Philadelphia, US) e.g. the relative standard deviation, also called the coefficient of variation, "CV". The `CV` of a value determined by a counting procedure will depend on the number counted. For a typical Poisson distribution the `CV` is inversely proportional to the square of the number counted, Thus, the uncertainty of a large number of particles or cells, e.g. somatic cells in milk will be smaller than the uncertainty of a small number of cells.
In typical, known flow cytometers, this is not taken into account. In these instruments, the number of cells is determined in a pre-determined volume of a milk/dye mixture. However, in some instruments, this is taken into account by performing an alternative measurement by detecting a pre-selected number of cells and by determining the volume in which this number of cells are present. Cell counting is also performed using techniques other than flow cytometry. Manual cell counting is often performed on human blood. In this technique, the cells in a pre-selected volume of the blood are counted. Alternatively a volume of blood containing a predetermined number of cells may be measured.
The above two methods have the disadvantages that they in a sense are static. Either the uncertainty is the same for all measurements or it is allowed to drift for all measurements. Therefore, there is a risk of obtaining results having a large uncertainty. The solution to this is to either increase the volume examined in order to count more particles or cells or to use the alternative method using a predetermined number of cells and therefore a pre-selected uncertainty. By this method, however, the measurement procedure may for some samples take an unacceptably long period of time.